Similar to an ultimatum, but one where the answer is truly "maybe" to both possibilities. In most cases an ultimaybe is much harder to deal with than an ultimatum due to it's confusing unanswerable nature.

Person A: You should really go to the dance with your girlfriend instead of that really pretty girl you're not actually in a relationship with.

Person B: But... I could be...

Person A: Doubtful.

Person B: Huh... this is an Ultimaybe. I'll never know what the right choice was, ever.

When you think you are giving an ultimatum, but there are really a whole lot of answers you will accept.

Person A: "I totally gave Bob an ultimatum yesterday when I said that he has to choose between me and his girlfriend, but he said he chooses us both, so we're all good."
Person B: "So it's really more of an ultimaybe."